1. Field of the Invention
A nondestructive method and apparatus for uniquely distinguishing among and quantifying the mass of individual nuclear fuel plates in situ in fuel drawers utilized in nuclear reactors is described. The method is both rapid and passive, reducing the personnel hazard of the commonly used irradiation techniques. It comprises simultaneous collimated neutron and collimated, energy resolved gamma ray analyses. The spatial resolution of both is sufficient to identify the smallest expected unit plate which is 2.5 cm long.
Said simultaneous neutron and gamma ray analysis is crucial to avoid intentional attempts to confound the procedure for the purpose of theft of fissile materials. The several pieces of information obtained for each fuel element make successful illegal substitution of materials in an attempt to deceive the system, virtually impossible, whereas, for example, simple neutron monitoring could be quite readily overcome. High spatial resolution of the present method also prevents the averaging of neutron or gamma events essential in the deception by substitution.
The invention is a result of a contract with the Department of Energy.
2. Description of Prior Art
The novelty search has produced several patents which differ from the instant invention in one or more of several ways. The most important and prevalent difference is that many of these patents teach neutron irradiation of the sample to be investigated with subsequent analysis of the resulting neutrons and gamma rays which arise from nuclear decomposition (U.S. Pat. Nos. 3,496,357 and 3,786,256 exemplify this approach.) The instant invention provides sufficient sensitivity to simply monitor the spontaneous decomposition of normally radioactive nuclei, thereby eliminating the hazard and inconvenience of neutron irradiation. The second salient difference is that those patents which do not actually mention neutron irradiation but suggest simultaneous neutron and gamma ray monitoring of samples do not teach energy resolved gamma analysis (See, e.g., U.S. Pat. No. 3,786,257.) A third group mentions either resolved gamma or resolved neutron analysis but not a combination of the two (See U.S. Pat. No. 3,717,765 for a description of the use of gamma spectroscopy, for example). A maximum of non-destructively and rapidly obtained sample information is crucial to the method of the instant invention which uses such analysis to quantitatively identify all radioactive elements present in nuclear fuel samples to reduce the possibility of willful deception by potential thieves.
The simultaneous, rapid use of both spatial resolving power along with isotope specific gamma and neutron materials identification of an unirradiated fuel sample is therefore unique. Further, the use of .sup.3 He proportional counters in a collimated neutron scanning system is novel by itself.